1. Field of the Invention
This invention relates to a new process for the production of polyisocyanates containing isocyanurate groups and to the use of the products obtained by this process, optionally in blocked form, as the isocyanate component in polyurethane lacquers.
2. Description of the Prior Art
There are several known processes for the trimerization of organic isocyanates (J. H. Saunders and K. C. Frisch, Polyurethanes Chemistry and Technology, pages 94 et seq. (1962)). Suitable trimerization catalysts are strong organic bases such as the alkaline metal salts of carboxylic acids, metal alcoholates, metal phenolates, alkali metal carbonates, tertiary amines, tertiary phosphines and the "onium" compounds of nitrogen and phosphorus and also basic heterocycles of these elements.
The use of quaternary ammonium hydroxides as catalysts for the trimerization of isocyanate groups has been repeatedly described. According to JP-P No. 601,337 (U.S. Pat. No. 3,487,080), quaternary ammonium hydroxides are used together with certain co-catalysts; the examples primarily describe the partial trimerization of aromatic diisocyanates, although non-aromatic diisocyanates are also exemplified.
The process according to EP-A-No. 10,589 is a further development of the process according to the above-mentioned Japanese patent. According to this prior publication, hydroxyalkyl-substituted quaternary ammonium hydroxides are used as catalysts for the trimerization of HDI. Using these catalysts, HDI can be trimerized without cloudiness. The disadvantage of this process is that the hydroxyalkyl ammonium hydroxides are very difficult to produce in colorless form and have to be used in relatively large quantities of up to 0.6%. Accordingly, the products obtained by the process, i.e. the isocyanurate polyisocyanates freed from excess starting diisocyanate, may possess a yellowish color.
EP-A-No. 47,452 describes the production of mixed trimers based on HDI and IPDI using comparatively large quantities of catalysts as can be seen from the examples.
Other known processes for the production of isocyanurate polyisocyanates based on HDI are also attended by serious disadvantages. GP-P No. 920,080, DE-OS Nos. 3,100,262, 3,219,608 or 3,240,613 describe processes for the trimerization of HDI using metal-containing catalysts and co-catalysts, such as phenols, alcohols or tertiary amines. The metal compounds can only be removed from the end products by very expensive processes, if at all, and can affect subsequent applications and also the stability of the end products. In addition, the use of co-catalysts containing active hydrogen atoms leads to secondary reactions consuming valuable isocyanate groups. The same also applies to the process according to EP-A-No. 155,559, in which ammonium salts of organic acids are used as catalysts together with large amounts of alcoholic compounds.
In the processes according to EP-A-Nos. 57,653, 89,297 and 187,105, organosilicon catalysts are used in comparatively large quantities. These compounds also cannot be completely removed from the end product and affect its use.
Accordingly, an object of the present invention is to provide a new process for the partial trimerization of the isocyanate groups of organic diisocyanates containing aliphatically and/or cycloaliphatically bound isocyanate groups which combines the following advantages:
The end products of the process are substantially colorless, i.e. have a color value (HAZEN) according to DIN 53,409 of less than 100.
The end products of the process are free from cloudiness and can be dissolved without cloudiness in standard lacquer solvents.
The end products of the process do not contain any metal ions.
The process can be carried out using minimal quantities of catalysts without the need for large quantities of co-catalysts which consume isocyanate groups.
It has now surprisingly been found that this object can be achieved by the process described in detail in the following.